Experimental investigations are carried out for the determination of heat transfer coefficients between corotating and stationary parallel disks with internal heat generation. The model considered simulates a typical induction motor with unequal losses in the stator and rotor. The data collected cover a range of Reynolds and Taylor numbers for various heat inputs. Quantitative assessment has been made for the increase in heat transfer with speed of rotation and coolant flow, air being the cooling medium. Temperatures along the axial, radial, and tangential directions are measured, and heat transfer coefficients at the radially diverging section are evaluated. The results are especially relevant to the thermal design of electric machines, and more particularly to radially ventilated induction motors. The results obtained indicate that present-day designs are conservative and that considerable saving in material can be achieved. © 1988.